Temperature logs have been used for many years in the oil and gas industry to obtain inflow and outflow information of selected intervals. U.S. Pat. No. 4,520,666 discloses a method of determining inflow into a well by determining the temperature in the well along the length of the inflow region by moving a logging sonde which is equipped with a temperature sensor up and/or down through the well inflow region.
More recently fiber optic Distributed Temperature Sensing (DTS) systems have become available to obtain temperature profiles along a well on a permanent basis. International patent application WO 01/04581 discloses the use of a fiber optic DTS sensor to determine the mass flow rates of produced fluids in a wellbore.
The temperature profile in an oil and/or gas production well is based on the difference between the geothermal gradient along the well and the inflowing fluids. In vertical or deviated wells a temperature contrast develops if two or more zones at different depths produce to the well. This temperature contrast is a function of the reservoir depth, the geothermal gradient and the thermal properties of the fluid and the well (casing, cement).
In a horizontal inflow region of an oil and/or gas productions well there is no geothermal gradient along the horizontal section. Differences can only be caused either by cooling due to gas expansion, known as the Joule Thompson effect, which occurs in gas reservoirs or oil reservoirs where gas comes out of solution during production, or when water is produced from a deeper aquifer (potential higher temperature) or from a nearby water injector (potential lower temperature).
International patent application WO 00/11317 and U.S. Pat. No. 6,497,279 disclose methods of monitoring production in an oil production well by means of an electrical heater cable and adjacent fiber optical DTS sensor, wherein the wellbore is heated by the electrical heater cable during production and the fluid flow is determined from the temperature profile measured by the fiber optical DTS sensor on the assumption that the fluid flowing from the formation to the wellbore lowers the temperature in the wellbore at the inflow locations.
US patent application US 2003/0140711 discloses a method for monitoring the velocity of fluids in a well wherein a point near the lower end of the well is cooled during a well shut in and the upward velocity of the cold spot is measured after restarting production by a fibre optical temperature sensing cable that extends along the length of the wellbore.